[Patent document 1] JP 2002-54934 A
There is known a technology to update map data in a map display apparatus which displays a map using map data (for example, refer to Patent document 1).
Methods of update of map data may include, for instance, a method of updating the whole map data stored in the map display apparatus, a method of updating only a difference of a new version of the map from the previous version of the map, and a method of updating an area covering a specific geographical area range in the map data.
In order to realize the update of the area, the map data may be divided into several unit areas as update units. In addition, the method of dividing the map data into several unit areas requires a consideration about treatment of roads or planimetric features bridging over a border of two unit areas.
The treatment of roads is explained as follows. A road bridging over a unit areas border is divided into the two links at the unit areas border. With respect to the two links posterior to such division, a first border node existing on the unit areas border is provided as a node connected to one end point of one link, whereas a second border node existing on the unit areas border is provided as a node connected to one end point of the other link. In such cases, the first border node and second border node belong to mutually different unit areas, but have the identical single position.
In this regard, however, when the first border node belonging to one unit area and the second border node belonging to the other unit area are provided as a single node existing on the single position of the border of the adjoining unit areas, the following disadvantage may occur in the case of an area update.
That is, when an update of the unit area which the second border node belongs to is executed, the position of the second border node may change. If changing, a position gap or separation resultantly arises in between the positions of the first border node and the second border node.
When such a position separation arises, there is a possibility that the road is viewed as if being disconnected at the position separation in displaying a map based on the map data. That is, the appearance of the map may worsen.
In addition, the treatment of the planimetric feature is as follows. A planimetric feature bridging over the border between the unit areas is divided at the border. The first border planimetric feature belonging to one unit area and the second border planimetric feature belonging to the other unit area are generated. The first planimetric feature occupies the portion of one unit area contained in the original planimetric feature, whereas the second planimetric feature occupies the portion of the other unit area contained in the original planimetric feature. At this time, the inside of the polygon which is formed by connecting the feature points of the first border planimetric feature is occupied by the first border planimetric feature. In contrast, the inside of the polygon which is formed by connecting the feature points of the second border planimetric feature is occupied by the second border planimetric feature.
However, when the planimetric feature which occupies the geographical area range over the unit areas border is thus divided into two border planimetric features, the following disadvantage may occur in the case of the area update. That is when updating of one unit area, the occupied area range of the first border planimetric feature and the occupied area range of the second border planimetric feature separate from each other at the unit areas border. As a result, the configuration appearing when the occupied area range of the original planimetric feature is displayed on the map may different from the actual one, thereby possibly causing distortion. That is, the appearance of the map may worsen.